Bumping this old topic any help please get in touch - also would it be easy to build a foorswitch for this or would anyone know an aftermarket replacement I could use?
(It took the liberty of moving this a more appropriate part of the forum).
Amp foot switches are pretty simple affairs. I'm no expert so I'll let others chime in but you could buy a similar footswitch for another amp and modify it as necessary. Or just use the box, repaint it to your liking and replace the innards with your own. The components won't cost you much.
Full schematics for the HD500s, including the footswitch, are available as PDF files here...